It is well known in the art that titaniferous ores and slags can be attacked with concentrated sulfuric acid to solubilize the titanium as part of a process for producing titanium dioxide pigment. When such an acid attack is carried out on, for example, an ilmenite concentrate to sulfate both the iron and titanium contained therein, the generally adopted procedure involves mixing the finely ground concentrate with concentrated sulfuric acid in vats and adding a small amount of water. The heat of dilution of the acid initiates the reaction while the exothermic nature of the reaction itself enables it to propagate through the charge. As the sulfation proceeds, the charge within the vat thickens and then rapidly solidifies. An air sparge through the slurry is resorted to as a means of maintaining a homogenous suspension and obtaining a porous charge which can be leached from the vat after solidification.
The paste-like consistency acquired by the slurry in the course of the reaction, and its subsequent rapid solidification present severe obstacles to the devising of a procedure for implementing the acid attack in a continuous manner. One prior suggestion for avoiding the materials handling problem is described in U.S. Pat. No. 2,098,026. Essentially the solution advocated there entails recycling some sulfated titanium product and mixing it with the incoming ore feed so that when the appropriate amount of acid is added to the mixture, the initial charge for the acid attack process is in the form of a wet granular feed rather than a slurry. This granular feed is then fed to a gas-fired kiln where it is kept agitated while the sulfation reaction takes place. With such a process the alleged improvement in materials handling is gained at the expense of the overall efficiency of the process since the recycling of substantial amounts of sulfated product limits the utilization of the kiln space thereby necessitating the use of a kiln which is larger than would otherwise be needed. Moreover, a substantial amount of the energy used in the process is wasted on heating and agitating the reacted material.
Various other techniques have been advocated for carrying out an acid attack continuously, a common feature of these processes being that they rely on some form of mechanical device to break up the charge continuously as sulfation and solidification thereof occurs. Thus, in U.S. Pat. No. 2,098,025 a first technique is described which involves preparing a slurry of the titanium concentrate and sulfuric acid, charging the slurry to a pre-heated vessel in which the reaction is initiated, and allowing the partly reacted slurry to overflow into a heated pan equipped with a rotating plow-share. Another technique described in that same patent comprises charging the pre-mixed concentrate slurry into a heated inclined rotating kiln which is equipped with stationary scrapers projecting from the kiln shaft towards its inner walls. The acid attack is initiated in the upper end of the kiln, and as the reacting mass travels down the kiln, it is continually scraped off the walls and broken up to be finally discharged as a granular powder.
A somewhat similar approach is described in U.S. Pat. No. 2,557,528 which also entails the use of a heated inclined rotary kiln into which pre-mixed slurry is fed. In this case the scraping and breaking up action is provided by a finned rod freely lying within the kiln. In U.S. Pat. No. 2,098,055, a plurality of parallel helicoidal screws rotating at different speeds are relied upon to break up the reacting charge of titanium concentrate and sulfuric acid. Yet another alternative technique, described in U.S. Pat. No. 2,098,054 involves carrying out the acid attack in a heated ball mill so that the reacting mass is continuously pulverized.
None of the above proposed techniques provides an entirely satisfactory solution to the problem of carrying out the desired acid attack efficiently inasmuch as the various mechanical devices, in addition to constituting capital expenses in themselves, inherently entail a wastage of energy used for continuously breaking up the solidifying mass.